An ancient inscription discovered on a 14th century church in Spain's Galicia region has been identified as Gaelic; the first written evidence of the northern region's Irish and Scottish heritage.

For centuries it has gone unnoticed, weathered by Galicia’s incessant drizzle but still visible to those with an eagle-eye.

On one of the granite walls of Santiago church in the small town of Betanzos, a small previously unintelligible inscription five metres above ground kept historians and epigraphists, or people who study ancient inscriptions, baffled for decades.

Researchers working for a private association called the Gaelaico Project now believe they've finally deciphered what it reads: "An Ghaltacht" or "Gaelic-speaking area".

"If our interpretation is right, the inscription isn't related to religious matters, but rather to the language that was spoken in Galicia at the time," Proxecto Gaelaico head Martín Fernández Maceiras told local daily La Voz de Galicia.

"It seems logical that the inscription was made while the church was being built (in the 14th century)."

Up to now, Galicia, along with Asturias and northern Portugal, have been informally considered part of the ancient Celtic nations (Ireland, Scotland, Wales, Brittany, Isle of Man and Cornwall) due to cultural and historical reasons rather than because of written proof.

Although researchers are hoping to get a second opinion from expert epigraphists on whether the inscription does indeed read “Gaelic-speaking area”, the chances of it being the first written evidence of Galicia’s Celtic past are high.

Despite the dominance of Latin, there are plenty of Gaelic traits still present in Galicia,” James Duran, academic expert on minority languages at the US's Stanford University, told La Voz de Galicia.